Perhaps he wants as few people as possible to trade on the exchanges right now. If people don’t keep their Bitcoins on the exchanges, they won’t be able to buy or sell Bitcoins at the best price.
Well, it is still depend whether you keep buying bitcoin offline or you are ready for the risk to used exchanges. It is right not to put big money in exchanges even if its a big one since everyone is vulnerable to be scammed and hacked. This is just for your own safety, so decide for the best.